Light-sensitive electric device including silicon



R. s. OHL 2,443,542

LIGHT-SENSITIVE ELECTRIC DEVICE INCLUDING SILICON June 15, 1948.

4 Sheets-Sheet 1 Original Filed May 27, 1941 wvg/vron R. 5. OHL

Maui 6 KM June 15, 1948. R. s. OHL 2,443,542

LIGHT-SENSITIVE ELECTRIC DEVICE INCLUDING SILICON Original Filed May 27, 1941 v 4 Sheets-Sheet 2 '92 SI IN SILICA CRUCIDLE IN ELECTRIC FURNACE IN VACUUM OR HELIUM ATMOSPHERE HEAT SLOIVLY T0 sou: P0/N1(e.9-I600'c) 400v: THE

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s. OHL

LIGHT-SENSITIVE ELECTRIC DEVICE INCLUDING SILICON Original Filed May 27, 1941 4 Sheets-Sheet 3 FIG.

FIG. 9

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Patented June 15, 1948 LIGHT-SENSITIVE ELECTRIC DEVICE INCLUDING SILICON Russell s. Ohl, Red Bank, N. 1., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Original application May 27, 1941, Serial No. 395,410, now Patent No. 2,402,662, dated June 25, 1946. Divided and this application September 17, 1942, Serial No. 458,709

9 Claims. (Cl. 136-89) This invention relates to lightsensitive electric devices and more particularly to photo-E. M. F. cells comprising fused silicon of high purity.

This application is a division of application Serial No. 395,410, filed May 27, 1941, issued as U. S. Patent 2,402,662, June 25, 1946, for Light sensitive electric device.

An object of the invention is to provide an improved light sensitive electric device.

Another object of the invention is to provide an improved method of making light sensitive electric devices of fused silicon of high purity.

In an example of practice illustrative of this invention, a photo-E. M. F. cell is formed of a portion of a silicon ingot which is provided with conductive terminals. The ingot is produced by fusing metallic silicon in powdered form in a silica (SiOz) crucible in an electric furnace and slowly cooling the fused material until it solidifies and for a period of time thereafter. The" powdered metallic silicon used is of a high degree of purity, say 99 per cent or higher. Certain material which has proved very satisfactory has a purity of approximately 99.85 per cent, Ingots which are suitable for the production of photo- E. M. F. cells possess a characteristic structure which is visible-when the surface is suitably prepared in vertical section. The upper portion of the ingot exhibits a columnar crystalline structure while the lower portion is non-columnar, and across the ingot in the lower section of the columnar portion is a striated zone, the striations extending across the ingot. This striated zone has the characteristics of a barrier zone or barrier layer and is conveniently designated simply a so-called barrier. The portion of the ingot suitable for photo-E. M. F. cells includes this barrier.

A portion of silicon is cut from the ingot in such a way that the barrier forms an extended surface backed by a relatively large amount of metallic silicon which was adjacent to the unexposed surface of the barrier. A low resistance conductive terminal is secured to the metallic surface remote from the barrier by plating with rhodium. Another conductive terminal is secured to the exposed barrier surface remote from the metallic silicon in such a way as to permit illumination of an appreciable portion of the exposed barrier surface. Circuit connections may be soldered to these terminals. The silicon adjacent to the barrier may have either the columnar structure or the non-columnar structure.

In another example of practice illustrative of the invention a photo-E. M. F. cell is formed by slicing off the top of an ingot which has a hump of extruded material in the upper surface of the ingot, the whole upper surface being covered with an active layer so that the top surface has a pale yellow or greenish fluorescent appearance. and attaching contact terminals such as a plating of rhodium on the metallic silicon surface remote from the active layer and a ring of sputtered platinum around the outer edge of the active layer. Circuit connections may also b soldered to these terminals.

The invention will now be described more in detail having reference to the accompanying drawing.

Fig. 1 shows in cross section an ingot of fused silicon Within a silica crucible from which ingot material for photo-E. M. F. cells according to this invention maybe cut;

Fig. 2 illustrates a photo-E. M. F. cell of another form cut from the ingot of Fig. 1;

Figs. 3 and 4 show the cell of Fig. 2 in one form of mounting;

Fig. 5 shows the cell of Fig. 2 in a modified form. of mounting;

Figs. 6 and 7 illustrate another form of mounting of the cell of Fig. 2 wherein a reflecting surface is employed;

Fig. 8 is an operational diagram of one form of the method employed for producing photo- E. M. F. cells some of the steps of which are used in making photo-E. M. F. cells in accordance with this invention;

Fig. 9 is a diagrammatic showing of a test arrangement for determining the longitudinal distribution of, the sensitivity of a photo-E. M. F. cell;

, Fig. 10 is a plot of data obtained with the arrangement of Fig. 9;

Fig. 11 is an arrangement, similar to Fig. 9, for determining the transverse distribution of the sensitivity;

Fig. 12 is a plot of data obtained with the arrangement of Fig. 11; Fig, 13 shows diagrammatically a test arrangement for determining the axial distribution of the sensitivity of a cylindrical section cut froman ingot having two barriers;

Fig. 14 is a plot of data obtained with the arrangement of Fig. 13;

Fig. 15 are plots of current, voltage and resistance, respectively, versus lumens of a typical photo-E. M. F. cell embodying this invention;

Fig. 16 is a plot of the spectral response of an other typical photo-E. M. F. cell embodying this invention;

Fig. 1'7 shows in cross section an ingot of fused silicon within a silica crucible in which ingot the top surface is electrically light sensitive;

Figs. 18 and 19 illustrate a form of photo- E. M. F. cell cut from the upper portion of the ingot of Fig. 17 according to this invention;

Figs. 20, 21 and 22 illustrate a modified photo- E. M. F. cell according to this invention in the fabrication of which the columnar material above the barrier and a small amount of light sensitive barrier material is removed; and

Figs. 23, 24 and 25 illustrate still another modification according to this invention in the fabrication of which the material on the non-columnar side of the barrier and some of the barrier material is removed.

Like elements in the several figures of the drawing are indicated by identical reference characters.

During an investigation of the production of fused silicon of high purity and its use for point contact rectifiers applicant discovered that under certain conditions this material was sensitive to visible light, generating an electromotive force independently of any applied voltage. The light sensitive effects were of a magnitude comparable to the most effective photoelectric substances then known.

The manner of the discovery was briefly as follows:

A considerable number of melts of pure silicon had been made up in connection with the abovementioned investigation. The material for some of these melts had been heated in a dry helium atmosphere. From each of a plurality of ingots resulting from some of these melts in helium a cylindrical rod had been cut for the purpose of making specific resistance measurements. These rods were about inch long and inch diameter. The rod from one of these melts had been equipped with metal end-pieces by a rhodium plating and lead-tin soldering process hereinafter to be described, to provide a goo-d connection for the specific resistance measurements. Such a measurement was being made on this rod by applicant when he noticed, while viewing on an oscilloscope the wave shape of the fill-cycle current flowing through the rod. that the current in one direction was afiected by light from an ordinary 40-watt desk lamp. A battery was then substituted for the GO-cycle current source and a rotating shutter was held between the rod and lamp to produce 20-cycle interruptions. A substantially square-top wave form was seen by applicant in the oscilloscope. Upon reduction and, finally, the elimination of battery voltage the square-top form persisted, although at a reduced amplitude.

This entirely unexpected phenomenon was recognized as of possibly great importance in the art of light sensitive electric devices and further study of this phenomenon was undertaken forthwith. The outcome of such study is that improved light sensitive electric devices and particularly photo-E. M. F. cells of high sensitivity and great stability have been made available. The present invention is a result of the abovementioned discovery.

A form of ingot from which photo-E. M. F. cells can be out is shown in Fig. 1. The ingot is [formed by the solidification of fused silicon in a silica crucible 6. Such an ingot made from certain kinds of highly purified silicon powder in a manner hereinafter to be described, comprises two zones of visibly different structure.

The upper zone 1 has a columnar structure, the columnar grains being of the order of one-half millimeter in width and extending down from the top of the ingot-to a distance of 5- or 10 millimeters. The lower zone 8 has a non-columnar structure. The ingot fractures most easily in the lengthwise direction of the columns. The columnar portion of the fracture appears lustrous while the non-columnar portion has the appearance of a grayish mass of smaller crystals. Across the lower portion of the columnar zone 1 some sort of boundary or barrier 9 is found. In this region 9 the columnar portion tends to be striated. the striations extendin glmross as well as between the columns. These striations appear, under a microscope, to have discontinuities at the columnar boundaries.

The above-mentioned barrier 9 is apparently the seat of the photo-E. M. F. effect. The upper zone 1 of the ingot 5 develops, on exposure to light, a positive potential with respect to the lower zone 8.

The photo-E. M. F. devices of Figs. 2 to 7, even though they are not being claimed in this application, will first be described to show the characteristics of the material used in the specific devices disclosed and claimed in this application. An understanding of these characteristics will facilitate an understanding of the invention of this application.

The photo-E. M. F. device of Fig. 2 comprises a silicon slab I0 cut from the ingot 5 of Fig. l at the position indicated by the dot and dash rectangle II. This rectangle H outlines the section of the slab lil midway between the edges'and parallel thereto. In other words, the slab I0 is so cut from the ingot 5 that the barrier 9 lies approximately midway between the ends of the slab.

The slab l0 may be out from the ingot 5 by any suitable process, but preferably by a process which conserves as much useful material as possible. -The uppermost and lowermost portions of the ingot may be used for other purposes, such as contact rectifiers. The intermediate portion, including the barrier 9, may be used for photo- E. M. F. cells. A metal wheel charged with diamond particles is suitable for cutting the ingot 5, a stream of distilled water being used to clear the cut particles from the kerf and to cool the surfaces. 1

The surfaces of the slab l0 wherein the outcropping of the barrier 9 occurs, may be used in the condition in which they are cut from the ingot 5. There is an advantage, however, in polishing these surfaces in order to facilitate transmission of the exciting light into the interior of the slab 10. These surfac s may advantageously be polished in many ways. One method which has been used is as follows The surface was first roughed fiat with 600 mesh Aloxite, or M-302 optical powder, using an iron lap followed by 1000 mesh Aloxite, and a lead lap in the subsequent polishing with an optical powder such as for instance No. optical powder. Suitable polishing abrasives are obtainable from the Norton Company, the American Optical Company or the Carborundum Company.

In order to facilitate the use of the slab I0 as a photo-E. M. F. cell, contact terminals l2 and I3 are provided on the ends of the slab by a process of rhodium plating. In a rhodium plating process which has been found to be very satisfactory, the end surfaces of the slab are ground fiat using a 600 mesh diamond wheel and water lubrication, Thereafter the end surfaces, includ and is highly resistant to corrosion. Suchcontacts are remarkably free from noise when used in communication circuits, such as are used to convey sound currents.

The size of the'ph'oto-E. M. F. cell or unit l of Fig. 2 is not-critical, but it has been found that advantageous dimensions are 11 millimeters for length, millimeters for width, and 0.6 millimeter for thickness. The barrier 0 lies advantageously about midway between the terminals [2 and II.

One arrangement for mounting the photo- E. M. F. cell of Fig. 2 is illustrated in Figs. 3 and 4. Two pairs of spring clips II and ii are secured to a block of insulation I by machine screws II. The photo-E. M. F. cell I0 is slipped betweenthe springs of the pairs of spring clips I and It with the contact terminals l2, and I3 in contact with the springs. Punched lips I! prevent the photo-E. M. F. cell I0 from sliding down 'too far. Conductors l0 and 20 are connected to clips M and II, respectively. When the barrier 0 of the photo-E. M. F. cell I0 is irradiated, a positive potential is developed in conductor I! with respect to conductor 20 providing that'the columnar end of the unit is in contact with clip II, as shown. I

Another arrangement for mounting the unit I0 such as illustrated in Fig. 2, is shown in Fig. 5. The unit I0 is provided with terminal conductors 2| and 22 by soldering. In soldering. the rhodium end surfaces It and I3 are tinned with ordinary lead-tin solder using an acidified zinc chloride flux. The solder must not be heated much above its melting point or there is danger of the rhodium being completely dissolved. The ends of the conductors 2| and 22 are freely tinned, then placed in contact with the respective tinned rhodium surfaces and the joint heated' until the solder flows, the excess solder being squeezed from between the conductor and the rhodium plating. A strong bond results. The unit III with the conductors 2| and 22 is then insulatingly mounted on a copper block 23. Victron or other suitable lacquer is used to secure the unit ll to the block 23 with a sheet of insulation 24, such as a sheet of cigarette paper, between the unit- In and the block 23. The conductors 2| and 22 areadvantageously held out of contactvwith the, copper block 23 by wrappings 25 and 25, respectively, of friction or rubber tape.

Still another arrangement for mounting the unit I. of Fig. 2' is illustrated in Figs.v 6 and-7 which is adapted to make use of reflected light. The unit II is provided with conductors ll and 32 in the manner described in connection with Fig. 5. The polished. faces of the unit iii are treated to reduce the surface reflection losses by the application of approximately a quarter wavelength thickness of "Victron" lacquer. Two dippings of the polished silicon surfaces in Victron lacquer is highly beneficial in improving thefresponse of these'photo-E. M.F. cells to light. The

coated unit i0 is insulatingly mounted on a copper block II, the surface of the block adjacent to the unit having been highly polished and advan- Meet? held out of contact with the copper block at bywra'lipings l4. ll, similar to those described inconnectionwithFig. 5. n

operational diagram ro producing a photo-E. MLF. unit'is shown in Fig. 8,- some steps of which are used for producing the photo- E. M. F. unit according to this invention. Silicon oi a purity in exces of 99 per cent obtainable in granular form is placed in a silica crucible in an electric 'f'umace in vacuum or helium atmosphere. Because of a tendency to evolution of gas with violent turbulence of the material. it isdesirable to raise the temperature to the meltin point by heating the charge slowly. Silicon will be found to fuse at a temperature of the order of 1400 to 1410" C.

In order to facilitate the heating process the silica crucible containing silicon may be placed within a graphite crucible which lends itself to development of heat under the influence of the high frequency field of the electric furnace to a much greater degree than does the silica crucible or its charge of silicon. Care must-be taken, however to avoid exposure of the melted silicon to graphite, oxygen or other materials with which it reacts vigorously. In this manner, the melt may be brought to a temperature of the order of 200 0. above melting point. In an example of practice of this process "high form" crucibles of. 50 cubic centimeter capacity obtainable from Thermal Syndicate Ltd., 12 East 46th Street, New York, New York, were employed. A furnace power input of 7.5 to 10 kilowatts was employed, the required time for melting being of the order of ten to twenty minutes,

then fell at the rate of about centigrade degrees per minute.

i in coolingthereisatendency after the upper surface solidified for extrusion ,of metal to occur through this surface during the-solidfication 'of the remaining material. Upon examination of the cooled ingot it isfound that a portionofthe grain structure is columnar, as hereinbeforeexplained. This is,.in "general, the upper portion. of fheingotfor the first material tosolidfy. Inthe are'alast to solidify and beyond the columnar grains anon-c'olumnarfstructure occurs, Between the zone first to cooland that to'coolth'ere is found to be some sort of'a boundary bummer which occurs in .a

plane normal to the columns'and this barrier has extremely important light'sensitive electric properties. The barrier ordinarily occurs a' short distance above where the columnar and non- 'coiumnar zones merge ;so that it extends across 7 the columns near their lower ends. The region above the barrier develops a positive thermoelectric potential with respect to an attached copper electrode and may, therefore, be designated as the P" zone. The region below the barrier develops a negative thermoelectric potential with respect to an attached copper electrode. It will be designated as the 11" zone.

To prepare a photo-E. M. 1". cell like that of Fig. 5 for example, a slab of material is cut from the ingot in such a manner as to be bisected approximately. by the barrier. The surfaces of the slab parallel to the barrier may be ground flat and electric terminal elements attached thereto in the manner diagrammed in Fig. 8 and described hereinbefore.

Granulated silicon of high purity now available on the market is produced by crushing material found in a large commercial melt. That supplied by Eiectrometalluii'gical Company, 30 East 42nd Street, New York, New York, is of a size to pass a 30 mesh screen and to be retained by an 80 mesh screen. The crushed material is purified by treatment with acids until it has attained a purity considerably in excess of 99 per cent. The chemical composition of a typical sample of this material is approximately:

Si 99.85 0 .001 C .019 H .001 Fe .031 Mg .007 Al .020 P .011 Ca .003 Mn .002 N .008

In some samples amounts up to .03 Ti and .004

amass is to be understood that these results are actual.

results obtained with certain specified photo- E. M. F. units.

Fig. 9 illustrates a test arrangement for determining the location and size of the photo- E. M. F. region in a rectangular slab of silicon cut from an ingot in such a manner that the barrier approximately bisects the slab intermediate the ends thereof. It was a simple matter to determine that the sensitive region was a strip across the face of the slab, probably only a few millimeters wide. This was done by moving a light spot over the face of the slab while the terminals were connected to a milliammeter mum current at a plurality of transverse positions. In order to determine the actual dimensions of the sensitive region pieces of opaque black paper were slotted with variable width slots and slid over-.the surface of the slab until a maximum response was obtained for a given intensity of illumination of the area exposed by the slot. in Fig. 9. The slab I00 provided with rhodiumplated soldered wire terminals IOI and I52 is connected to a microammeter I03. A sheet of black paper I00 covers the slab except for the surface exposed by slot I05 which lies transversely across the slab I00. In order to find the position of maximum current response with a given width of slot I05 and given intensity of illumination of the surface of the slab exposed by the slot, the surface of the slab is explored by moving the paper I with the slot I05 lengthwise across the slot the slab. lbr each width of slot the response would vary as typified by the graph of Fig. 14. but would differ from the specific shape there shown dependent upon the location of the barrier and the width of the slot. With acertain slab-type of photo-E. M. 1'. cell. mounted as shown in Fla. 5, which is 11.4 millimeters long, 5.5 millimeters wide and 0.6 millimeter thick. the data of Fig. 10 a was obtained. The slot width is plotted as ordinates and the current at the pomtion of maximum response as abscissae. The curve I shows that the response is linear for small slot widths up to about 1.5 millimeters but that beyond this width there is a relatively small increase of response current from 14 microamperes to 18 microamperes for the whole length of the slab which. as mentioned above, was 11.4 millimeters. Therefore, it can be said that for this particular slab a strip of illumination about 1.5 millimeters wide across the slab yields very nearly e total response for a given light flux intensity.

Fig. 11 shows a test arrangement much like that of Fig. 9 but adapted to determine any variation of the photo-E. M. F. region transversely of the slab. The black paper Ill with I05 is rotated 90 degrees with respect to its position in Fig. 9. The data of Fig. 12 was obtained with various slot widths oriented as in Fig. 11 and with the same photo- E. M. F. cell as described above which cell is 11.4 millimeters long, 5.5 millimeters wide and.0.8 millimeter thick. At the position of maximum response for each slot width the response is proportional to the width of the light band as shown by curve I01 of Fig. 12. This shows that there is negligible variation in the photo-E. M. I". region transversely of this slab.

It has happened that an ingot was formed with a double barrier one of which was;near the top of the ingot and the other near the bottom. A photo-E. M. F. cell cut from such an ingot and including both barriers exhibits a double peaked response when explored with a narrow light spot.

'65 and noting the positions of the spot for maxi- One such piece of paper is illustrated A test arrangement for determining such response is shown in Fig. 13. Such "a photo-E. M. F. cell H0, provided with metal terminals III and III, is connected to an ammeter III. A small transverse strip of the cell is illuminated by light from a source Ill directed by lens II5 through an aperture H5 in a. sheet of black paper III. The cell H0 is moved lengthwise in front of the aperture-il5 and the current deflection in the meter H3 is observed. With a certain rod-type of photo-E. M. F. cell, designated rod No. 2, which is 3.15 millimeters in diameter, '30 millimeters long and 24 millimeters between the plated terminals, the data of Fig. 14 was obtained. The width of the light beam was approximately one-half millimeter and its wave-length was 1.1 microns. The current deflection is plotted as ordinates and the distance from one selected end in centimeters is plotted as abscissae. As shown by curves III and H0, this photo-E. M. F. cell exhibits two maxima of current response which are of opposite polarity. If both barriers are illuminated simultaneously the effects are opposing in the series circuit including the meter IIJ. In this particular rod, one barrier is more responsive than the other, as indicated by the height of the peaks of curves 0 and H0. The width of the areas under response curves III and I I9 is probably due to the fact that the pure silicon of these units is appreciably light transmissive.

The illumination-response characteristics of the photo-E. M. F. cell which is hereinbefore described in connection with Figs. 9 to 12 as being 11.4 millimeters long. 5.5 millimeters wide and 0.6 millimeter thick are shown by the curves of Fig. 15. The light source used to obtain this data was a 2l-candlepower 6- to 8-volt automobile lamp No. 1130 operated at 7.40 volts and a, color temperature of 2930 K. The filament was focused in a spot at the barrier at a magnification of about unity. The flux in this beam was varied by inserting Levy line screens whose transmission was determined in the position used. Curve I20 shows the open-circuit voltage and curve I2I shows the short-circuit current dependence on incident light flux in lumens. The voltage is expressed in volts and the current in milliamperes. Curve I22 shows the value of resistance at which the short-circuit current was reduced to one-half.

The spectral sensitivity of a cylindrical type of single-barrier photo-E. M. F. cell is shown in Fig. 16. This cell is 3.15 millimeters in diameter, 26 millimeters long and 21 millimeters between the terminal platings. Curve I25 shows the spectral sensitivity. The ordinates are equienergy values, the maximum being unity. The abscissae are wave-lengths of the light in microns. The measurements were made using a small spot of light from the spectrometer focused on the barrier. It will be noted that, while the cell has some sensitivity in the visible region, the maximum is out in the infra-red. Since the light penetrates an appreciable thickness of silicon to reach portions of the barrier, the optical transmission of the silicon has an effect on the shape of curve I25. Because of'this large response in the deep infra-red, the silicon photo- E. M. F. cells of this invention provide a tool not available heretofore in the optical art.

During the production of silicon ingots according to the method hereinbefore described it was discovered that in a small proportion of the melts, say 3 to per cent, the top of the melt was covered with material which was extruded from the interior during the cooling process. The top surfaces of some such ingots had a. pale yellowish and greenish fluorescent appearance. It was discovered by applicant that, if a contact were made to the top surface of such an ingot and some other point of the ingot, an electric current would flow if the top of the ingot were irradiated with infra-red or visible light. The electrons are apparently very efllciently released by the light and driven into the main conducting body of purified silicon. Substantially the full sensitivity is developed whatever may be the size of the surface contact area. This type of photo-E. M. F. cell shows some response for ultra-violet radiation.

An ingot which shows extruded material and the above-mentioned surface layer is shown in vertical section in Fig. 17. The ingot I30 is formed by the solidification of fused silicon powder of high purity in a silica crucible I3I. The extruded material forms a hump I32 in the upper surface of the ingot, while the-whole surface is covered by the active layer I33. The thickness of this layer I33 is necessarily greatly exaggerated in Fig. 17. It is in fact of microscopic thickness.

A photo-E. M. F. cell according to this invention comprising this sensitive layer I33 may be I fabricated by slicing off the top of ingot I30 contact for the sensitive surface layer comprises a ring of sputtered platinum I35. Another way to form a contact ring is to use platinum paint and heat or fire the unit at a temperature of 500 to 550- C. The sensitive surface layer is not injured by such firing operation. The other contact I36 attached to the body of silicon I31 may be formed as hereinbefore described by electroplating with rhodium to which a copper meter MI by means of conductors I38 and I30.

The illumination of the light sensitive layer is indicated by the arrows I42. The whole unit may be dipped in Victron or other suitable lacquer. The thickness of the lacquer on the sensitive layer may advantageously be made onefourth the wave-length of the energizing radiation. Such a thickness reduces reflection as hereinbefore described in connection with Figs. 6 and 7.

Another form of photo-E. M. F. cell according to this invention in which the surface of the barrier is illuminated, is illustrated in Figs. 20, 21 and 22. Fig. 22 is a cross section through the cell of Fig. 21 at the plane indicated by the arrows 23, In fabricating one form of such a cell, the columnar zone of. a body of fused silicon is cut away down to the barrier, terminal contacts being made to the resulting exposed surface of the barrier zone and the opposite surface of the noncolumnar zone. As shown in Fig. 20, a block of silicon I50 in the form of a parallelepiped is cut from a silicon ingot such as that shown in Fig. 1, having a barrier zone 9 substantially parallel to thetop and bottom faces of the block. The columnar zone I is at the top and the noncolumnar zone 8 at the bottom. The lower surface of the non-columnar zone 8 is provided with an electrical contact I5I by plating with rhodium in the manner hereinbefore described. The upper portion of the columnar zone I is then cut away down to the plane represented by the line X--X. This plane extends through the upper layer of the barrier zone, the location of which zone is determined by exploring the vertical surfaces of the block I50 with a small spot of light and noting the current response in a test circuit connected to the upper and lower surfaces of the block I50. The resulting barrier surface I52 is then highly polished and a narrow strip around the periphery of the polished surface is roughened with M-302 emery. Both the polished and roughened portions of surface I52 are then plated with rhodium in the manner hereinbefore described. The rhodium plating is then rubbed off from the polished portion of the surface I52 leaving a strip of plating I53 around the periphery. This strip I53 serves as an electrical contact for the barrier region of the resulting photo-E. M. F. cell. Conductors I54 and I55 may be soldered to the rhodium contacts in the manner hereinbefore described. The illumination of the light sensitive surface is indicated by the arrows I55. When illuminated, conductor I55 assumes a positive potential with respect to conductor I54.

Another photo-E. M. F. cell, similar to that just described but in which the non-columnar material 8 is cut away down to the barrier 9. is illustrated in Figs. 23, 24 and 25. Fig. 25 shows a cross section of the cell of Fig. 24 at the plane indicated by arrows 3 I. In fabricating this form 1 1 of cell, a block of silicon I80 similar to that of Fig. 20, having a columnar zone I, a non-columnar zone 8 and a barrier region 9 is cut from an ingot of fused silicon. The bottom surface of the columnar zone I, the columnar zone being at the bottom in Fig. 23, is provided with an electrical contact IGI by plating with rhodium in the manner hereinbeiore described. The noncolumnar zone 8 is cut away down to the upper portion of the barrier region represented by the plane indicated by the line YY. The resulting barrier surface I 62 is polished, roughened around the periphery and plated with rhodium which is partly removed to form contact I63 as explained in connection with the cell of Figs. 21 and 22. Conductors I64 and I65 are soldered to the rhodium plating IBI and I63, respectively. The polished surface I62 is shown illuminated by light rays I55 passing through filo-degree lenses I". When illuminated, conductor I64 assumes a positive potential with respect to conductor I65.

The edges of the cells of Figs. 22 -and 25 may be provided with an opaque insulating coating, such as black pitch, to exclude extraneous light. The -degree lenses of Fig. 25 may also be used with the cell of Fig. 22. The use of such lenses is advantageous when these cells are used ,as exposure meters.

It has been found that individual photo- E. M. F. cells of the kind illustrated in Figs. 22 and 25 are about equally sensitive over the whole exposed barrier surface as determined by exploring the surfaces of several cells with a small spot of light. It is advantageous to cut close to or even into the barrier region. Measurements made on several cells indicate that there is some advantage in cutting away the columnar region of the silicon block as illustrated in Figs. 20, 21

, and 22 instead of the non-columnar region as illustrated in Figs. 23, 24 and 25. The columnar material is more transparent than the non-columnar material and accordingly a somewhat larger proportion of the light can reach the light sensitive region. However, both types of cells are useful. The amount of silicon adjacent to the barrier region on the side opposite to that which is illuminated, in these types of cells appears not to be critical.

An ammeter I 51 is shown connected to the conductors I54 and I55 of the photo-E. M. F. cell of Fig. 22 and an ammeter I68 is shown connected to conductors I 64 and I65 of the photo- E. M. F. cell of Fig. 25.

A coating of Victron or other suitable lacquer may be used on the polished surfaces of the cells of Fig. 22 and 25 to reduce reflection losses.

The nature of the boundary or barrier zone and the reasons for its electrical behavior are obscure. There is evidence to indicate that the phenomena observed are dependent not only upon high purity of the silicon but also upon the character of the extremely small amounts of impurities which remain. In the most satisfactory ingots the N" zone portions have very tiny gas pockets and upon cutting through this zone a characteristic odor of acetylene is observed. Moreover, certain lots of highly pure silicon which have at first appeared to be defective in barrier-forming properties have been satisfactorily conditioned by the introduction of carbon or silicon carbide into the melt in amounts of the order of 0.1 per cent to 0.5 per cent and this should be done if a preliminary sample of a particular lot of material does not form the distinctive barrier structure.

' have been etched and stained. The barrier zone is evident as one or more striations of a somewhat different appearing material in consequence of its different reaction to the etching acid. In the case of slow cooling the striation extends across the entire ingot, thus dividing the ingot into discrete P and N" zones. Where, however, the cooling is precipitate as in the case of shutting oil! the heating power suddenly as soon as fusion occurs and permitting the temperature to fall suddenly, the first spots to cool develop P zones and these are surrounded by N zone matrices in such irregular fashion as to render the resulting ingot quite unsatisfactory for photo-E. M. F. cells. The slow coolingrate is important in developing an orderly striation or barrier. Features of the method of preparing effective silicon materials are described and claimed in the application of J. H. Scaif, Serial No. 386,835, filed April 4, 1941, issued as U. S. Patent 2,402,582, June 25, 1946, for Improvements in the preparation of silicon materials. For further information regarding material from which light sensitive electric devices according to this invention may be fabricated, reference is made to the disclosure of this Scaif patent.

"Victron lacquer, which has been referred to hereinbefore, is a solution of the polystyrene with the addition of a small amount of resin to produce It is a commercial overlying metallic silicon, a conductive terminal adapted to permit illumination of an appreciable portion of said top surface in contact with said top surface, and another terminal in contact with the metallic silicon.

2. A photo-E. M. F. cell comprising a body of silicon of high purity produced by cooling fused silicon powder of a purity in excess of 99 per cent, a surface layer of material extruded from the fused silicon after the surface has solidified, and electrical terminals connected to said body of silicon and said layer, respectively, the terminal connected to said layer being adapted to permit illumination of an appreciable portion of said layer.

3. A photo-E. M. F. cell comprising the upper portion of an ingot of fused silicon the top surface layer of which portion comprises material formed from material from the melt having a yellowish and greenish fluorescent appearance overlying metallic silicon, terminal metal sputtered on said top surface and adapted to permit illumination of an appreciable portion of said top surface, and an electrical contact connected to said metallic silicon.

4. A photo-E. M. F. cell which comprises a body of solidified fused silicon of high purity including a striated zone, said body being cut from an ingot having a striated zone between a columnar znrm zone and a light sensitive barrier zone in the vicinity between the P and N zones, removing a body of material from said ingot including integrally connected P zone, barrier zone and N zone material, determining the location of the outcropping of said barrier zone on the exposed surfaces of said body by exploring said surfaces with a small spot of light and noting the current response in a test circuit connected to the P zone and the N" zone material, respectively, removing the material of said body of material on one side of said barrier zone subsequently to the determination of the location of the barrier zone as indicated by said outcropping of said barrier zone to expose a surface which was adjacent to and covered by the removed material before its removal, and connecting electrical contacts to the surface of said barrier zone which was exposed by the removal of said material and to the surface of the remaining material remote from said barrier zone, respectively, the contact connected to the barrier zone being adapted to permit illumination of an appreciable portion of the exposed surface of said barrier zone.

6. A photo-E. M. F. cell comprising the upper portion of an ingot of fused silicon the top surface of which ingot comprises material having a yellowish and greenish fluorescent appearance overlying metallic silicon, said material having been extruded from the fused silicon after the surface had solidified, a conductive terminal adapted to permit illumination of an appreciable portion of said top surface in contact with said top surface, and another terminal in contact with the metallic silicon.

7. A photo-E, M. F. cell comprising the upper portion of an ingot of fused silicon having a purity in excess of 99 per cent, the .top surface of which ingot comprises material having a yellowish and greenish fluorescent appearance overlying metallic silicon, said material having been extruded from the fused silicon after the surface had solidified, a conductive terminal adapted to permit illumination of an appreciable portion of said top surface in contact with said top surface, and another terminal in contact with the metallic silicon.

8. A photo-E. M. F. cell comprising a body of solidified fused silicon of a purity in excess of 99 per cent including a striated zone, said body being cut from an ingot having a striated zone between a columnar zone and a non-columnar zone and the striated zone being adapted to be exposed to radiation at right angles thereto, an electrical contact connected to the surface of said striated zone remote from said other zone and adapted to permit radiations to reach an appreciable portion of the surface of said striated zone, and an electrical contact connected to a surface of said other zone remote from said striated zone.

9. A photo-E. M. F. cell comprising an integral mass of high purity solidified fused silicon formed by cooling a melt of high purity silicon, said mass having a body portion and a layer-like portion in part adjoining each other, an electric contact connected to a part of said body portion which is remote from said light sensitive portion, and an electric contact connected to the surface of said light sensitive portion remote from said body portion and adapted to permit illumination of an appreciable portion of the surface of said layerlike portion, said layer-like portion being formed of material extruded from the melt during cooling, which material is more light sensitive than the material of said body portion.

RUSSELL $1031..

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,037,713 Allen Sept. 3, 1912 1,998,334 Rupp- Apr. 16, 1935 2,061,357 Heyroth et al. Nov. 17, 1936 OTHER REFERENCES Coblentz, W. W. Bureau of Standards Sc. Papers, vol. 16 (1921), pages 608-0. 

